The present invention relates to a method and an apparatus for positioning a movable element, in particular a rotating mirror, according to a predetermined course of desired position values using a galvanometer actuator to which a control unit provides a control signal.
In the prior art the control unit has been designed as a PID (proportional plus integral plus derivative) controller. Such a controller determines the control signal for the element from the position signal of a position sensor, by generating the deviation between the desired position and the actual measured position, the integral of such deviation, and their time derivative being used as input parameters for the controller. The disadvantage of this way of controlling is often inadequate accuracy in effecting rapid changes in the desired position signal.
U.S. Pat. No. 5,450,202 discloses a system to position a mirror by means of a galvanometer actuator operated in resonance mode and in which a control unit regulates the current through the actuator such that the mirror occupies with maximum accuracy a defined position at control times half the resonance period apart. The sequence of desired positions recurs cyclically, i.e. is the same in every run. The current flow between two consecutive control times is held at a constant level calculated by the control unit. The algorithm is designed to be adaptive in the sense that the deviation between the actual position and the desired position is recorded for every run based on information provided by a position sensor, and this deviation is used to calculate the value of the current for the desired position in the next run and minimize deviations. This correction is accomplished with a parameter derived from a model that considers the mirror and the actuator as a damped harmonic oscillator. This model is not adaptive. The disadvantage of this system is that the control strategy only works for systems running in a resonant mode in which a course of certain desired position values is repeated cyclically. On the one hand, the position can only be influenced at certain predetermined resonance times, and on the other hand the system does not provide an adequately accurate control for a course of set values that runs only once.
Patent publication JP-A 10023777 discloses a system for positioning a mirror the control unit of which uses a motion model of the mirror actuator to calculate the control signals using filters for the angular acceleration and the current of the motor.
Patent publication DE-A-43 44 283 discloses a control method for an electric motor that uses a model with an adaptive parameter determination.
Patent publication U.S. Pat. No. 5,233,512 discloses a control method for an electric motor that uses a model of the electric motor to control the error rate of the control method.
Patent publication U.S. Pat. No. 5,519,605 discloses a control method using a process model based on a step response or an impulse response.
The task of the present invention is to provide a method and an apparatus for positioning an element which enable the tracking of an arbitrary course of desired position values by the positionable element with maximum accuracy, using a minimum of calculation effort but nevertheless being suitable for high dynamic requirements.
The advantage of this invention is that the selected model, the selected control strategy, and the use of a pre-filter for checking and, if necessary, optimizing the course of desired position values allow an exact realization of this course of desired position values within the scope of the system while using little calculation effort and satisfying high dynamic requirements.